1. Field of the Invention
The present invention relates to a bump forming method. More particularly, the present invention relates to a method of forming metal bumps on electrodes provided on a printed circuit board, a wafer or a ceramic board by utilizing a resin film as a mask.
2. Description of the Related Art
Recently, there is an increasing demand for mounting electronic components on a substrate (e.g. a printed circuit board) at high densities. For meeting such a demand, much attention is focused on bear chip mounting. The bear chip mounting includes the face-up bonding which utilizes wire-bonding for providing electrical connection between a chip and a wiring pattern on a circuit board, and the face-down bonding which utilizes metal bumps for providing electrical connection. Recently, the face-down bonding increasingly replaces the face-up bonding.
The face-down bonding which utilizes metal bumps is capable of connecting electronic components at a low resistance. In forming metal bumps, the following requirements should be satisfied.
When the electrodes of an electronic component are arranged at a high density, metal bumps need be correspondingly arranged at a small pitch to be precisely positioned on the electrodes. This is particularly true in forming metal bumps on the electrodes of a semiconductor device. Further, all metal bumps need to have an equal height for ensuring reliable connection between electronic components. In addition, there is also an inherent demand for a decrease in the manufacturing cost.
Conventionally, metal bumps for face-down bonding have been formed by plating or vapor deposition. However, the formation of metal bumps by such methods requires much cost for the equipment while providing difficulties in controlling the height and composition of the bumps. Recently, therefore, the conventional methods are increasingly replaced with a metal mask printing method and a resin mask loading method for realizing a cost reduction while providing a higher freedom in controlling the composition of metal bumps.
FIGS. 2a through 2e illustrate a prior art metal mask printing process for forming metal bumps. According to the metal mask method, as shown in FIG. 2a, use is made of a board 20 provided with electrodes 21, and a metal mask 22 formed with openings 22a corresponding to the electrodes 21. As shown in FIG. 2b, the metal mask 22 is placed on the board 20 for bringing the openings 22a in conformity with the electrodes 21. Then, as shown in FIG. 2c, a solder paste 2325 containing solder powder is loaded in each of the openings 22 by printing. Then, as shown in FIG. 2d, the metal mask 22 is removed from the board 20. Subsequently, as shown in FIG. 2e, the solder powder contained in the solder paste 23 is melted by heating, thereby providing generally spherical metal bumps 24 on the electrodes 21 of the board 20. The metal bump formation by such a metal mask printing method is disclosed in JP-A-7-302972 for example.
FIGS. 3a through 3e illustrate a prior art resin mask loading process for forming metal bumps. First, as shown in FIG. 3a, a resin film 32 is formed on a board 30 provided with electrodes 31. Then, as shown in FIG. 3b, the resin film 32 is partially etched away for forming openings 32a for exposing the electrodes 31 of the board 30. Then, as shown in FIG. 3c, a solder paste 33 containing solder powder is loaded in each of the openings 32a. Subsequently, as shown in FIG. 3d, solder powder contained in the solder paste 23 is melted by heating, thereby providing generally spherical metal bumps 34 on the electrodes 31 of the board 30. Finally, as shown in FIG. 3e, the resin mask 32 is removed from the board 30.
The metal mask printing method described above has a drawback that it has difficulties in controlling the height of the metal bumps formed at a relatively small pitch. Specifically, when the openings 22a of the metal mask 22 are arranged at a small pitch, removal of the metal mask 22 may cause part of the solder paste 23 filled in the openings 22a to be removed together. As a result, the metal bumps 24 may vary significantly in height, which may hinder reliable mounting of electronic components.
In the resin mask loading method, on the other hand, the resin film 32 as a printing mask is removed after the solder paste 33 are melted by heating. Therefore, it is possible to reliably form each of the metal bumps 35 with a predetermined amount of solder paste even when electrodes are arranged at a small pitch. Thus, in comparison with the metal mask printing method, the resin mask loading method is preferable for forming metal bumps at a small pitch which is necessary for realizing high density mounting of electronic components.
However, the prior art resin mask loading method has the following problems. In melting the solder powder contained in the solder paste 33 in the step shown in FIG. 3d, the solder powder is generally heated at a temperature which is 30xcx9c50xc2x0 C. higher than the melting point of the solder metal. In this heating step, however, the resin film (typically made of a thermosetting resin) hardens to some extent under heating. Therefore, in removing the resin film in the subsequent step shown in FIG. 3e, part of the resin film thus hardened may remain on the surface of the board, which hinders reliable mounting of electronic components.
It is therefore an object of the present invention to provide a method of forming metal bumps using a resin film, which is capable of easily removing the resin film for forming good bumps which allow reliable mounting of electronic components.
According to a first aspect of the present invention, a method of forming metal bumps is provided which comprises the steps of: forming a resin film on a surface of a substrate provided with electrodes; forming openings in the resin film for exposing the electrodes; loading a bump material into the openings, the bump material containing a metal material which melts only partially at a first temperature, the metal material melting entirely at a second temperature higher than the first temperature; heating the bump material to the first temperature for melting only part of the metal material; cooling the bump material below the first temperature; removing the resin film; and heating the bump material to the second temperature for entirely melting the metal material.
With the above-described method, it is possible to remove the resin film from the surface of the substrate more reliably than in the prior art method. Specifically, when the bump material loaded in the openings is heated to the first temperature (hereinafter referred to as xe2x80x9cprimary heatingxe2x80x9d), only part of the metal material changes from solid phase to liquid phase. At this time, due to surface tension, the liquid phase part tends to cohere for integration with the remaining solid phase part. When the bump material is thereafter cooled below the first temperature, the liquid phase part returns to solid phase for fixing to the electrodes (hereinafter referred to as xe2x80x9cprovisionally fixingxe2x80x9d). The cooling herein includes natural cooling wherein no positive cooling using a coolant is performed. According to the present invention, the resin film formed on the surface of the board as a mask is removed after the provisional fixing.
In the prior art method, since the bump material is heated immediately to such a high temperature as to completely melt the metal material contained therein, the resin film hardens undesirably due to such heating, consequently leading to difficulty in removing the resin film. According to the present invention, by contrast, the bump material is provisionally fixed to the electrodes by the primary heating followed by subsequent cooling, and the resin film is removed before the solder material is heated at the second temperature (hereinafter referred as xe2x80x9csecondary heatingxe2x80x9d) which is higher than the first temperature. Therefore, the resin film can be removed more easily than in the prior art method. With no resin film left on the substrate, electronic components can be reliably mounted on the substrate via the bumps.
In a first embodiment, the metal material comprises a metal alloy of a composition which has a solid-liquid coexistent temperature range between a solidus temperature and a liquidus temperature. In the first embodiment, the first temperature is equal to or higher than the solidus temperature and lower than the liquidus temperature, whereas the second temperature being equal to or higher than the liquidus temperature.
Herein, the solidus temperature and the liquidus temperature are defined as follows. Under a given pressure, the solidus temperature of an alloy is a temperature below which the alloy exists only in solid phase, whereas the liquidus temperature of an alloy is a temperature above which the alloy exists only in liquid phase. At temperatures including and between the solidus temperature and the liquidus temperature, the solid phase and liquid phase of the alloy coexist.
In a second embodiment, the metal material contains a plurality of different metals. In the second embodiment, one of the metals has a lowest melting point, whereas another of the metals has a highest melting point. Further, the first temperature is equal to or higher than the lowest melting point and lower than the highest melting point, whereas the second temperature is equal to or higher than the highest melting point.
Herein, the melting point of a metal is the xe2x80x9cmelting pointxe2x80x9d in its normal sense for a mono-elemental metal. For an alloy, on the other hand, the melting point means the liquidus temperature of that alloy under a given pressure.
According to the second embodiment, it is possible to individually select said one metal for melting under the primary heating and said another metal for melting under the econdary heating, so that the composition of the resulting umps can be easily controlled.
According to a second aspect of the present invention, method of forming metal bumps is provided which comprises the steps of: forming a resin film on a surface of a board provided with electrode; forming openings in the resin film for exposing the electrodes; loading a bump material into the openings, the bump material containing a metal of a composition which has a solid-liquid coexistent temperature range between a solidus temperature and a liquidus temperature; heating the bump material to a first temperature which is equal to or higher than the solidus temperature and lower than the liquidus temperature; cooling the bump material below the solidus temperature; removing the resin film; and heating the bump material to a second temperature which is equal to or higher than the liquidus temperature.
According to a third aspect of the present invention, a method of forming metal bumps comprising the steps of: forming a resin film on a surface of a substrate provided with electrodes; forming openings in the resin film for exposing the electrode portions; loading a bump material into the openings, the bump material containing a plurality of different metals, one of the metals having a lowest melting point, another of the metals having a highest melting point; heating the bump material to a first temperature which is equal to or higher than the lowest melting point and lower than the highest melting point; cooling the bump material below the lowest melting point; removing the resin film; and heating the bump material to a second temperature which is equal to or higher than the highest melting point.
The substrate used in the present invention may be a silicon wafer or a circuit board formed of glass-fiber-reinforced epoxy resin. The substrate is provided, at predetermined portions thereof, with a plurality of electrodes made of copper, nickel, or gold for example.
The resin film may be made of a photosensitive resin such as an acryl-based, epoxy-based or imide-based resin or a combination of these resins. The resin film may be etched by utilizing the known photolithography including light-exposure and development.
Alternatively, use may be made of a non-photosensitive resin. In such a case, etching may be performed by the application of a laser beam.
The resin film may be separately prepared and attached to the substrate. Instead, the resin film may be formed in situ by applying a liquid resin on the substrate. For forming high bumps on the electrodes at a small pitch, it is preferable that the resin film has a thickness of 30xcx9c150 xcexcm.
The resin film may be removed with a stripping agent. Examples of the stripping agent include strong alkali such as an aqueous solution of sodium hydroxide, organic alkali such as an aqueous solution of monoethanolamine or an aqueous solution of tetramethylammonium hydroxide. In use, the stripping agent may be mixed with an additive which preferably has a function of breaking the resin film into small pieces to prevent the resin from remaining on the board.
Preferably, the bump material may be in the form of a paste prepared by kneading metal powder with a flux. The flux may be a mixture of rosin, a thixotropic agent, an activator, and a solvent for example.
The bump material may contain an alloy consisting of at least two metal elements selected from the group consisting of Sn, Pb, Ag, Sb, Bi, Cu, In, and Zn for example. Specific examples include Sbxe2x80x94Sn alloy, Snxe2x80x94Bi alloy, Snxe2x80x94In alloy, Snxe2x80x94Pb alloy, Snxe2x80x94Ag alloy, Snxe2x80x94Cu alloy, Snxe2x80x94Zn alloy and Snxe2x80x94Pbxe2x80x94Sb alloy. More specifically, 5% Sn-95% Pb alloy, 43% Sn-57% Bi alloy or 48% Sn-52% In alloy may be used. Alternatively, the bump material may contain a mono-elemental metal such as Sn, Pb or In.
Rosin has a primary role of increasing tackiness of the solder paste. Specifically, use may be made of polymerized rosin, hydrogenated rosin or esterified rosin.
The thixotropic agent has a primary role of providing the solder paste with shape-holding ability. Examples of the thixotropic agent include hardened caster oil and stearic amide.
The activator has a role of removing an oxide film or the like formed on the surfaces of the solder particles and/or on the surfaces of the electrodes when the solder paste is heated. With the aid of the activator, the surfaces of the solder particles and/or the surfaces of the electrodes can be cleaned, which enhances adhesion of the solder to the electrodes and thereby enables reliable formation of good metal bumps. Organic acid and/or organic amine may be used as the activator. Examples include sebacic acid, succinic acid, adipic acid, glutaric acid, triethanolamine, and monoethanolamine.
The solvent has a role of melting soluble components and making the flux vehicle into a pasty state. Preferably, a solvent having a boiling point which is close to or higher than the melting point of the solder may be used alone or in combination with another such solvent. Examples include higher alcohols and glycol ethers such as diethylene glycol dimethyl ether, n-buthyl phenyl ether, 2-methyl-2, 4-pentanediol, and diethylene glycol monobutyl ether.
Preferably, each of the openings formed in the resin film for loading the solder paste has an area no more than 25 times the area of a corresponding electrode. This makes the solder collect reliably on each of the electrodes upon melting for reliably forming a spherical bump.
Other features and advantages of the present invention will become clearer from the detailed description given below with reference to the accompanying drawings.